New imaging technique shows freezing pattern in plants

Imaging that shows how ice forms in oats could help breeders develop hardier varieties and expand their range.

Oct 28, 2014

Using a new technique to study an old problem, a United States Department of Agriculture’s (USDA) Agricultural Research Service (ARS) scientist in North Carolina has uncovered new details about what happens to a cereal plant when it freezes.

Agronomist David Livingston has developed an imaging technique and recently used it to show that when an oat plant freezes, ice forms in its roots and in portions of its crown, which lies just below the soil surface and connects the roots to the stalk. The results have implications for growers.

Outside view of an oat crown reconstructed in 3D from 186 images taken through a light microscope.

In winter cereals like oats (Avena sativa), the crown is where the plant generates new tissue growth—if it survives the winter cold. Oats won’t grow in many northern areas because of cold temperatures. Understanding how ice forms in oats could help breeders develop hardier varieties and expand their range, Livingston explained.

The process involved making high-resolution digital photos of standard histological slices of plant tissues and using commercially available software to create a three-dimensional perspective, which gave added depth to their structures, above and below ground. The resulting images were similar to those produced by magnetic resonance imaging (MRI) and computed tomography (CT) scans, but can be created from much smaller tissue samples than what CT and MRI tests require. They are also less expensive to produce, because they require less expensive equipment and training.

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Livingston’s studies have so far focused on oats because their production in the U.S. is limited by their sensitivity to subfreezing temperatures. But, he has also used the technique to examine wheat, barley, rye, and corn, and he said it could be used to study other crop plants. The technique even works on mammalian systems and has been used to produce three-dimensional reconstructions of tumors in liver biopsies.

In the oats study, Livingston stained frozen tissue samples and took 186 sequential images of them with a digital camera. He then aligned the images and used imaging software to clear away the background colors so he could focus on cavities formed by ice crystals in the crown tissues of the oats. He compared the images from frozen plants with images from plants kept at normal temperatures.

Along with showing how ice forms in the root, the images revealed that ice formation in the crown is limited to its lowest and uppermost parts, apparently leaving the middle free of ice—at least free from crystals big enough to visualize. The ice also didn’t form in the shape of circular crystals, as portrayed in two-dimensional images. Instead, the crystals were shaped more like elongated curtains.